The present invention relates to hybrid active power filters. More specifically, the present invention relates to hybrid active power filters having programmed impedance characteristics for improved filtering effectiveness.
A hybrid active filter is a combination of a passive filter and an active filter. Such filters are well suited for use in reducing harmonic distortion in power systems which may result from a variety of industrial and power system equipment, e.g., high-voltage direct-current (HVDC) power transmission systems and motor speed controllers. This harmonic distortion is undesirable as it is often the source of telephone interference (i.e., the distortion signal is coupled onto the telephone lines running along side of the power lines), excessive heating of electrical equipment on the power system and other distortion related problems.
Passive filters were originally employed to filter the harmonic distortion in power systems, whereby a passive filter was connected in parallel with the load or harmonic source on the system to provide a low impedance path for removing the distortion currents. With the recent availability of high power inverters, active filters have been considered for filtering the harmonic distortion in power systems. In prior art active filter systems, distortion currents are measured on power system and are used for controlling the active filter. Typically, the active filter includes a power electronic converter that either supplies or draws distortion currents to remove distortion currents from the power system.
Parallel combinations of shunt connected active and passive filters are known. see, e.g. a paper entitled "New Trends in Active Filters for Improving Power Quality", by Hirofumi Akagi, IPCC-95-59 ("Akagi paper"). Harmonic reduction is achieved by measuring load current and injecting a compensating current, thereby resulting in a corresponding reduction in source current harmonics.
Combinations of a shunt connected passive filter with an active filter in series with the power source are also known, see, e.g., Akagi paper. Harmonic reduction in these combinations is achieved by measuring source load voltage and current on the power, system, and injecting a compensating voltage in series with the source, thereby isolating load bus harmonics from the source.
Series combinations of active and passive filters connected in shunt with the power system are known, see, e.g., Akagi paper. Harmonics reduction is achieved by measuring power system source current and driving a compensating voltage in series with the passive filter, thereby diverting load harmonics into the passive filter.
Combinations of an active filter connected to inject a current in parallel with one or more elements of a passive filter are known, see, e.g., U.S. Pat. No. 3,849,677 entitled "Hybrid Power Filters Employing Both Active and Passive Elements". Harmonic reduction is achieved by measuring source current or voltage on the power system and driving the compensation current in parallel with the passive filter, thereby diverting load harmonics into the passive filter.
U.S. Pat. No. 5,548,165 entitled "Hybrid Filter for Reducing Distortion in a Power System" discloses a hybrid filter for reducing distortion in a power system. The power system includes a voltage source connected to a load through power lines. An inductor which has a low impedance at the fundamental frequency of the power system and a high impedance at other frequencies is connected to a capacitor which has a high impedance at the fundamental frequency and a low impedance at other frequencies. An active filter is connected in parallel with the inductor. The voltage rating of the active filter at the fundamental frequency is significantly reduced as compared to applying the active filter alone. However, the series connection of the inductor and capacitor may interact with the impedance of the power system, possibly resulting in a series-resonance or a parallel-resonance condition. In order to prevent such resonance, the active filter is operated to provide damping at the resonant frequency in addition to supplying the required distortion current. The active filter provides damping by simulating resistance at the resonant frequency. Specifically, the active filter is operated to draw a current component in phase with the voltage at the resonant frequency. This requires that the active filter draw a small amount of real power from the power system at the resonant frequency. The active filter absorbs this power on the DC side of the active filter. Power for the DC side is supplied from a separate source.
A controller controls the active filter. The controller receives signals proportional to the power system voltage across the load and the voltage across the active filter from potential transformers. A signal proportional to power system current drawn by the load is provided by a current transformer, while a second current transformer provides a signal proportional to the current drawn and supplied by the active filter. Accordingly, the active filter is operated such that it draws a distortion current component which causes all of the load distortion current to flow through the capacitor. Thus, no distortion current flows through the voltage source. Also, the active filter draws a current in phase with the voltage across it to damp any resonance condition.
It will be appreciated that all of the above discussed active filters employ a power system line level reference (i.e., sensed current and/or voltage at the load and/or source of the power system) for closed-loop control. A disadvantage of sensing current and/or voltage at the load and/or source of a power system is that devices for sensing such are exposed to the full-line voltage and/or current of the power system, whereby expensive, high power rated sensing devices are required. Also, logistical problems can arise with respect to the physical distance between and/or the location of the active filter's control device and the required sensing devices. Moreover. employing closed-loop control at the power system line will likely result in control instabilities when modifications are made to the power system itself. Another disadvantage of the aforementioned prior art is the problem of regulating current between multiple active filters, each sensing the same power system line. In such a configuration, each active filter will be trying to compensate for the other.